1. Field of the Invention
The present invention generally relates to solid-state image sensing apparatuses, and more particularly, to a solid-state image sensing apparatus provided with an adjusting function for white balance, and a white balance adjusting method.
2. Description of the Art
In general, color image sensing with the use of a solid-state image sensing device such as CCD (Charge Coupled Device) has been implemented by provision of a color separating filter on an image sensing portion of a CCD. Meanwhile, a CCD camera for specific application has implemented color image sensing by using a light source of periodic emitting-type. The periodic emitting-type light source emits monochromatic lights such as red (R), green (G) and blue (B) repeatedly one for every field. An object is illuminated by this periodic emitting-type light source and sensed for its image by the CCD having no color filter mounted thereon, from which picture signals are obtained to be combined for every field. In such a manner, color image sensing has been implemented.
FIG. 1 is a block diagram showing a structure of a solid-state image sensing apparatus using a periodic emitting-type light source as described above.
A timing control circuit 7 generates, based on a basic clock CK, a horizontal scanning signal (or horizontal synchronizing signal) HD, a vertical scanning signal (or vertical synchronizing signal) VD, a sampling pulse SP synchronized with the vertical scanning signal VD, and a read timing signal FT. The read timing signal FT gets a read transfer pulse at a specific timing during the respective blanking periods of the vertical scanning signal VD. A light source 8 for illuminating an object is synchronized with the vertical scanning signal VD to emit the monochromatic lights R, G and B repeatedly each for one V period (one period of the vertical scanning signal). The light source 8 is constituted as being provided with a rotating filter plate in the optical path of white light illuminating an object, which comprises color filters of red, green and blue, and is synchronized with the vertical scanning signal VD for rotation.
A CCD solid-state image sensing device of the frame transfer type (referred to as CCD hereinafter) 1 comprises a photosensitive portion I, a storage portion S and a horizontal register H. This CCD 1 is driven by a drive circuit 6. The drive circuit 6 comprises a read transfer pulse generating circuit 6F for generating a read transfer pulse .phi..sub.F, a storage transfer pulse generating circuit 6S for generating a storage transfer pulse .phi..sub.S and an output transfer pulse generating circuit 6H for generating an output transfer pulse .phi..sub.H.
These pulse generating circuits 6F, 6S and 6H operate based on the basic clock CK from the same oscillation source for generating the transfer pulses .phi..sub.F, .phi..sub.S and .phi..sub.H, respectively.
The photosensitive portion I photoelectrically converts the light reflected back from the object for every one picture frame. Picture information (charge signals) obtained from the photosensitive portion I are read out in response to the read transfer pulse .phi..sub.F, to be first transferred to the storage portion S. The picture information stored in the storage portion S is transferred to the horizontal register H in response to the storage transfer pulse .phi..sub.S with one scanning line for a single horizontal blanking period. The picture information having been transferred to the horizontal register H is outputted as a picture signal Y in response to the output transfer pulse .phi..sub.H.
The picture signal Y is controlled in gain at an AGC circuit 2 before entered in sample and hold circuits 3R, 3G and 3B. The sample and hold circuits 3R, 3G and 3B sample and hold the picture signal Y for every vertical blanking period, based on the sampling pulse SP. This permits the picture signal Y to be separated in color component signals R, G and B, and then entered in gain control amplifiers 4R, 4G and 4B, respectively. The gain control amplifiers 4R, 4G and 4B are responsive to gain control signals CR, CG and CB for controlling the color component signals R, G and B in level to apply them to a signal processing circuit 5. The signal processing circuit 5 performs gamma correction, pedestal control and the like to output the color component signals R, G and B.
Light emission LC of such a light source 8 as described above is, as shown in FIG. 2, synchronized with the vertical scanning signal VD to take colors of red R, green G and blue B repeatedly one for every one V period. The photosensitive portion I is driven by the read transfer pulse .phi..sub.F so that a charge signal can be read out during a blanking period of the vertical scanning signal VD. Respective periods between one reading of a charge signal and the subsequent reading of another charge signal are regarded as photoelectric conversion periods E.sub.R, E.sub.G and E.sub.B. These photoelectric conversion periods E.sub.R, E.sub.G and E.sub.B correspond to the respective monochromatic lights R, G and B of the light source 8. Picture information having been obtained during the respective photoelectric conversion periods is read out in the subsequent fields, producing a picture signal Y. Therefore, in the picture signal Y, color components R, G and B appear by turns repeatedly, with one field (lV period) as a unit of the turn, to be outputted with a delay of one field with respect to the light emission LC.
The picture signal with the color components R, G and B appearing field by field is, as described above, sampled and held by the sample and hold circuits 3R, 3G and 3B sequentially for every one field to be separated in color component signals R, G and B.
When white balance of the picture signal is to be adjusted in the above solid-state image sensing apparatus, the color component signals R, G and B are individually changed in level by changing the gain control signals CR, CG and CB to be entered in the gain control amplifiers 4R, 4G and 4B, based on the light source.
However, when there are excessive differences between the levels of color component signals R, G and B, the gains of the gain control amplifiers 4R, 4G and 4B for the respective color components must be changed considerably in order to adjust white balance. This may disturb the balance of S/N ratio of the color component signals R, G and B, possibly resulting in degraded picture quality of a reproduced picture plane.